When using an array of light emitting diodes (LEDs) to illuminate a document within a scanner, there are a number of problems to overcome due to the ‘point source’ nature of LEDs versus the ‘continuous source’ nature of a fluorescent lamp.
One problem associated with the point source nature of LEDs is the effect of specular reflection, particularly at the document edge. The effect of the specular reflection may also be found in the centerfolds of glossy documents, however, the effect may be prominent at the document edge as the edge curves away from the image bearing surface and the glossy documents reflect light back into the sensor. The effect of specular reflection occurs where a glossy surface directly reflects the light from an illumination source onto an imaging sensor. With a continuous illumination source such as a lamp, this specular reflection may be seen as a white stripe in the cross-process (or fast scan) direction at the edge of the document. However, an array of point illumination sources, such as LEDs, may create a periodic pattern of white marks 200 at the document edge. This periodic pattern of white marks 200 is more objectionable in comparison with the continuous white line from a lamp. FIGS. 1A and 1B show images used to highlight the effect of specular reflection. FIG. 1A shows an image scanned with the continuous source of illumination, such as a lamp, where as FIG. 1B shows an image scanned with the point source of illumination, such as LEDs. The specular reflection can be seen on the left hand side of each image, where the fold in the spine of the document is located.
One solution to reduce the effect of “specular reflection” is proposed and is discussed in detail in U.S. patent application Ser. No. 12/014,361, incorporated herein by reference. This solution used a different light guide design, such as collimator lenses, to collimate the light from the LED and hence to reduce the effect of “specular reflection”.
Another problem associated with point source nature of LEDs is a lack of color uniformity. The solution proposed in U.S. patent application Ser. No. 12/014,361 does not address or solve the lack of color uniformity.